Ethylene terephthalate copolymers modified with dimethyl 5-sodiosulfoisophthalate or 5-sodiosulfoisophthalic acid are described in copending U.S. patent application Ser. No. 588,168 filed Mar. 9, 1984, now U.S. Pat. No. 4,499,262 by Fagerburg and McFarlane. These copolymers have a lower "planar stretch ratio" (described hereinafter) than unmodified polyethylene terephthalate at the same inherent viscosity. Thus, a polymer with a lower inherent viscosity can be used for the production of articles such as beverage bottles, and the polymer can be produced at a much faster rate than the unmodified polymer. When the aromatic modifiers are used, however, a buffer such as sodium acetate must be used to limit the formation of uncontrolled amounts of diethylene glycol as a by-product. The presence of sodium acetate (NaOAc) significantly reduces the polymerization rate and serves to offset the economic advantage of using these aromatic modifiers.
In the present invention, the alicyclic sulfonates used as modifiers do not require the use of buffers, and the modified polyethylene terephthalate has acceptable levels of diethylene glycol and acceptable polymerization rate.
The term planar stretch ratio is generally defined as the product of the stretch ratios of each of two directions of stretch (machine direction and transverse direction in film or sheet formation or axial and hoop directions in bottle formation). Thus, for a film stretched three times in each direction (machine and transverse direction or axial and hoop direction) so that the final lengths are three times the initial lengths, the planar stretch ratio would be 3.times.3 or 9. Critical planar stretch ratio is the planar stretch ratio at the point in the stress strain curve of a material at which strain hardening begins. It is in the design and manufacture of parisons from which containers such as beverage bottles are made that the critical planar stretch ratio is important. Thus, by multiplying the critical planar stretch ratio by the bottle-wall thickness desired the parison thickness necessary to make a bottle having that wall thickness can be determined. It therefore follows that if a material to be used in making parisons can be obtained so as to have a critical planar stretch ratio lower than that previously employed, the parison could be designed thinner and longer. The advantages to the container manufacturing industry in using parisons having thinner walls would be readily apparent. Because heat transfer is known to be directly related to the square of the thickness of the molded article, the time ordinarily required in molding articles will be reduced with the use of thermoplastic material having a lower critical planar stretch ratio. A reduced molding cycle time in turn results in increased productivity and decreased costs for the manufacturer.
While it is known that lower critical planar stretch ratios for certain thermoplastic materials such as poly(ethylene terephthalate) can be achieved by employing a material having a higher inherent viscosity (I.V.), this method is disadvantageous since a longer production rate or a greater number of reactors is required to obtain the desired higher I.V. Moreover, the increased process time makes obtaining a material with acceptable color more difficult since polyester materials are subject to yellowing, the degree of yellowing directly correlated with reaction time. Thus, an alternate means for obtaining thermoplastic materials such as poly(ethylene terephthalate) having a lower critical planar stretch ratio would be advantageous to bottle producers and other containers manufacturers.